What TimeBench does
- Clock check: how far your device clock is ahead or behind the exact time, in milliseconds, with the accuracy of the measurement (usually ±10–50 ms).
- Exact time: a large clock with your device’s error already corrected, next to the device’s own clock.
- Reference values: UTC, Unix time in seconds and milliseconds, your time zone and its UTC offset, the next daylight-saving change, the day of the year and the ISO week.
- Stopwatch with laps (the fastest and slowest are highlighted) and a countdown timer with sound and vibration.
- History: every measurement is kept in your browser, so you can see whether the clock drifts from day to day; any result can be shared as a link or a picture.
How to check your clock
- Open TimeBench. The measurement starts by itself and takes 2–8 seconds.
- Read the verdict under the big clock: exact, or how much the clock is behind or ahead.
- Check the accuracy in the details. If it is wider than you need, press Measure again on a calmer connection (Wi-Fi instead of mobile data, no downloads running).
If you change the device’s clock while the page is open, TimeBench notices the jump and measures again on its own.
How to read the result
| Colour | Difference | What it means |
|---|---|---|
| Green: “exact” | within the accuracy, or under 20 ms | the clock is synchronised; nothing to do |
| Green | up to 0.5 s | fine for everyday life, calls and online games |
| Amber | 0.5–2 s | noticeable for live broadcasts, auctions, ticket sales that open at a set second |
| Red | more than 2 s | automatic time is probably switched off; see below how to fix it |
Offset is the exact time minus the device time. A positive offset means the device is behind, a negative one that it is ahead. Accuracy (±) is the honest range of the measurement: the true difference is certainly inside it.
If the difference is almost exactly a whole number of hours or half-hours, the clock on the screen may look right while the time zone is wrong; TimeBench points this out.
How the offset is measured
TimeBench works the way NTP, the internet’s time protocol, does. It sends up to ten tiny requests to Cloudflare’s time-stamping server (speed.cloudflare.com) and notes when each one left and when the answer came back. The server writes its own time into the answer, so the true time at the server is somewhere between the two moments. The offset is the server time minus the middle of that trip, and the uncertainty is half the round trip.
The server gives its time in whole seconds, so TimeBench times each next request to arrive exactly when the server’s second is predicted to tick over. Whether the answer shows the old second or the new one halves the remaining uncertainty, and after 6–10 requests it shrinks to about half of the fastest round trip. The fastest trips are the most trustworthy, because they leave the least room for delays.
If Cloudflare cannot be reached, TimeBench asks the fof.lol server for the time from its Date header in the same way. That is only as accurate as that server’s own clock.
How to set your clock right
- Windows: Settings → Time & language → Date & time, turn on Set time automatically and press Sync now.
- macOS: System Settings → General → Date & Time, turn on Set time and date automatically.
- Android: Settings → System → Date & time, turn on Set time automatically (the name differs slightly between makers).
- iPhone and iPad: Settings → General → Date & Time, turn on Set Automatically.
Then press Measure again. A computer that drifts by seconds a day even with automatic time on may have a weak motherboard battery or a time service that does not run; the history shows the drift between measurements.
Stopwatch and timer
Stopwatch
Start, stop and resume as often as you like; Lap notes a split while it runs, and Reset clears it after a stop. With three laps or more the fastest is shown in green and the slowest in red. Copy laps gives a table you can paste into a spreadsheet. A running stopwatch keeps going if you reload the page.
Timer
Set hours, minutes and seconds or tap a preset from 1 minute to 1 hour. The ring shows the time left and the card shows when the timer will end. When the time is up you hear 30 seconds of beeps and the phone vibrates until you press Stop alarm; the alarm stops by itself after a minute. While the timer runs, the screen does not go dark (Wake Lock).
Why background tabs matter
To save battery, browsers slow down timers in tabs you are not looking at: Chrome runs them at most once a second, and after five minutes at most once a minute; phones may pause a hidden page altogether. Tools that count time by adding up timer ticks fall behind because of this.
TimeBench does not count ticks. The stopwatch and timer read the monotonic performance.now() clock and redraw with requestAnimationFrame, so after any pause they show the right time at once. The timer’s alarm is scheduled ahead on the audio clock, which keeps running in the background. Vibration and the screen still wait until you return, and on phones the alarm sounds only while the page is open, so keep TimeBench in front for anything important.
Privacy
The measurement requests carry no data about you and no cookies; the server only learns that someone asked for the time, plus the IP address any website sees. Results, laps and timer settings are stored only in your browser. A share link contains just the measurement: the offset, its accuracy, when it was taken and your time zone.
Frequently asked questions
How accurate is TimeBench?
Usually ±10–50 ms, and it always shows the exact range. On a fast, stable connection it can reach a few milliseconds; on a slow mobile connection the range is wider, but the result is still correct within it.
Why does the result differ a little between measurements?
Each measurement has its own accuracy range. Results that overlap within their ranges agree. If they do not, the device clock itself has moved, for example after a system time sync.
Does it work offline?
The stopwatch and timer work without a connection once the page is installed or open. The clock check needs the internet; without it the big clock shows the device’s own time.
Why is my phone clock wrong if it sets the time automatically?
Phones often take the time from the mobile network, which is not always precise; some sync only once a day or after a restart. A difference of under half a second is normal.
Can I use the timer with the screen off?
On a computer the alarm sounds even in a background tab. On phones the browser may pause the page when the screen is off, so the alarm will sound when you open it again; for cooking or sport, keep the screen on.